A water pump is used conventionally in a cooling device for a water-cooled engine installed in a vehicle or the like, and a cooling performance of the engine is closely related to a flow rate of a coolant circulated by the water pump. This type of cooling device is constituted by a coolant passage including a water jacket, which is provided in an engine main body, and a radiator, a thermostat, the aforesaid water pump, and so on, which are connected to the coolant passage. In this type of cooling device, the water pump is operated when the engine is driven to circulate the coolant through the coolant passage. As the coolant flows through the water jacket, heat exchange is performed with an engine main body, and as a result, the engine is cooled (see Patent Document 1, for example).